1. Field
The present disclosure relates to coherent detection of pulse position modulated signals. More particularly, the present disclosure describes a method and apparatus using a semiconductor optical amplifier (SOA) for demodulating optical pulse position modulated signals.
2. Description of Related Art
Many satellite and terrestrial optical communication systems require transmission of analog optical signals. The straightforward way to address this need is to modulate the amplitude of an optical carrier. This approach, however, suffers from poor signal-to-noise ratio (SNR). It is well known that broadband modulation techniques, which utilize higher bandwidth than that of the transmitted waveform, may improve SNR over that achieved with amplitude modulation. Pulse Position Modulation (PPM) is one of these techniques. In PPM, a temporal shift in the pulse position represents a sample of the transmitted waveform. The improvement in SNR near the Nyquist sampling frequency of a pulse position modulated signal over an amplitude modulated signal is shown below:SNRppm∝SNRam(tp/τ)2 where tp is the temporal spacing between unmodulated pulses and τ is the pulse duration.
Conventional detection or demodulation of analog PPM optical signals, though, suffers from poor SNR at low frequencies. PPM signals are usually demodulated from the optical to electronic domain by a photodiode followed by a lowpass filter (LPF) that converts pulse position modulation to amplitude modulation. Such a demodulation technique is not capable of recovering the DC component, since the DC component is represented by a constant temporal shift of all pulses from their unmodulated positions. Moreover, the demodulated signals after the lowpass filter have very low amplitude at low frequencies. The amplitude increases linearly with frequency up to the Nyquist limit. Such frequency-dependent distortion is corrected by an integration circuit, which amplifies low-frequency noise accordingly, resulting in decreased SNR performance.
An apparatus and method for detecting an optical PPM signal are described in U.S. Pat. No. 6,462,860, issued to Ionov on Oct. 8, 2002. This patent application describes coherent wavelength converters that are used to generate preferably top hat shaped optical pulses. Top hat shaped optical pulses are preferred in the apparatus and method described in U.S. Pat. No. 6,462,860, since such pulses provide for better linearity of the detected optical PPM signal. Top hat shaped optical pulses may also be used in other optical systems known in the art. U.S. Pat. No. 6,462,860 describes the generation of top hat shaped pulses with non-linear optical mirrors (NOLMs).
Another embodiment of a top hat pulse generator suitable for use in optical pulse position modulation detection is described in U.S. patent application Ser. No. 10/341,689, filed on Jan. 13, 2003 and entitled “An Optical Top Hat Pulse Generator.” The apparatus described in U.S. Ser. No. 10/341,689 uses NOLMs that are controlled by first order solitons. Still another embodiment of a top hat pulse generator suitable for use in optical pulse position modulation detection is described in U.S. Provisional Patent Application Ser. No. 60/488,540 filed on Jul. 18, 2003 and entitled “Method and Apparatus for Optical Top-Hat Pulse Generation.” The embodiment described in U.S. Ser. No. 60/488,540 also uses an NOLM controlled by first order solitons, but the NOLM also comprises one or more sections of polarization maintaining fiber coupled to each other at 90 degree angles.
While optical PPM receivers based on optical top-hat pulse generation provide outputs with the desired linearity, such NOLM-based receivers have drawbacks due to their complexity. The NOLMs may require careful balancing and adjustments to achieve the desired linearity. Further, the NOLM-based receivers may also require a number of erbium-doped fiber amplifiers (EDFAs) and optical filters with flat dispersion. These components increase the complexity and cost of such NOLM-based receivers. Therefore, there is a need in the art for a PPM receiver with less complexity than those PPM receivers known in the art.